1. Field of the Invention
The present invention generally relates to machine controls and more particularly to the control of combustion in a burner for heating water or other substances by controlling air flow into the burner responsive to changes in physical parameters affecting air and or fuel density.
2. Background and Description of the Prior Art
Burners for machine systems such as water heater boilers for example, generally mix a fuel in gas or liquid form with air to provide a source of heat. Efficient combustion occurs when (a) the ratio of the mass of air to the mass of fuel is held within a small range of values centered on approximately 18-to-1, and (b) sufficient air is mixed with the fuel to ensure combustion of all of the fuel plus some small amount of “excess air.” Generally, sufficient air is provided when the amount of excess air is approximately 15%, which corresponds with an air-fuel ratio of approximately 18-to-1. If the excess air exceeds about 15%, some of the heat produced is consumed heating the excess air and is thus not available for heating the water in the boiler. Thus, it is important to maintain a stable and relatively low excess air level.
However, unless the burner is operated in an atmosphere of substantially constant air temperature and barometric pressure, the setting of operating controls for the burner is at best only a rough approximation to an optimum level for efficient combustion over normal variations in temperature. Thus, these settings require a substantial offset to compensate for changes in the air temperature. The result is that excess air values often exceed the 15% figure by a wide margin, to as much as 30% or more, when the combustion air temperature changes, placing an extra burden upon the heat energy produced upon the burner. Such a situation may occur, for example, when the temperature may vary as much as 20° F. to 30° F. or more over a 24 hour period, or as much as 80° F. to 100° F. through seasonal variations. To compensate for such variations, some burner efficiency, and some fuel consumption, is traded off for ensuring complete combustion at all times to minimize unburned fuel and emissions.
Most burners built today use a “Volume Control” system to control the flow of fuel and air. On gas fueled burners, the fuel pressure is controlled with a regulating valve, and the correct flow rate is obtained with an orifice. The orifice may be fixed for “On-Off” firing or it may be a control valve (like a butterfly valve) which can be opened and closed to allow more or less fuel in. The combustion air is controlled in a similar manner, using a fixed orifice for “On-Off” air flow control and an air damper for modulating air control.
Conventional volume control systems for water heater burners are subject to errors in the control of the air and fuel rate because the correct proportions of air and fuel are defined by the mass flow not volume flow. For each pound of natural gas provided to the burner, a corresponding quantity of air is required (about 18 pounds of air). According to the gas laws, the mass provided by a given volume of air can vary according to its temperature and the barometric pressure. Thus, the ratio of mass to volume is defined as the density of a gas, and can be defined mathematically for our purposes as,Actual Density=(Std. density)×(absolute pressure/std pressure)×(std temperature/absolute temperature),  {Eqn. 1}where:
Density=weight of gas per unit volume of gas (lb/ft3 of gas at the stated pressure and temperature), and
Std. density=density of the gas at standard conditions (0.0765 lb/ft3 for air at 60° F. and 29.92″ Hg), where:
Absolute pressure=gauge pressure+barometric pressure of the current condition;
Std pressure=standard pressure, 29.92″ Hg (barometric pressure);
Std temperature=standard temperature, 60° F.; and
Absolute temperature=460+the temperature in ° F. of the gas.
These changes in density can result in large changes in the air-fuel ratio and the excess air of the burner combustion. For example, a difference of a combustion air temperature change from 120° F. on a hot afternoon to 40° F. on a cool morning will result in an increase in excess air of about 14%. This means that the burner is passing through 14% more excess air at 40° F. than at 120° F., and heating this air from 40° F. to the stack temperature (which is often around 500° F.) requires proportionately more fuel. This significantly reduces the efficiency of the boiler-burner package, making it more expensive to operate.
Oil fueled systems are not subject to the same density variations as a gas fuel system, because the liquid oil has a very small change in properties with temperature and pressure. For oil firing, the temperature generally must be controlled to maintain good atomization. Moreover, the oil pressures are so much higher than atmospheric pressure that the change in atmospheric (i.e., barometric) pressure has little effect. The concept of density change can be applied to oil flow, but it offers a much smaller improvement.
The impact of temperature and pressure variation is seen in the limitations and alternate control methods and systems used by burner manufacturers. Following are listed some typical methods that burner manufacturers use to solve these problems.                a. The simplest means of handling this is to allow for higher rates of excess air in the burner, and especially on cold days, set up the burner with very high excess air rates so that when it gets hot, there is enough air available to completely burn the fuel. This may typically be described in the service manual as a basic setup requirement.        b. Require the room to be heated to minimize combustion air temperature variations.        c. Perform more frequent burner tune ups, especially on a seasonal basis, to correct for some of the variation in the combustion air temperature.        d. Add an Oxygen Trim system to compensate for these changes by measuring the excess air and adjusting the fuel or air flow rate to obtain a constant excess air level.        e. Applications with outdoor installation or ducted outside air are generally required to have this air heated to reduce the variation in temperature to minimize combustion stability problems.        f. Add a fully metered control system. This system measures the mass flow of air and fuel. It is a very expensive option and rarely used.        
The concept of a “Fully Metered System” or “Full Metered Cross Limited Control System,” as described in (f) above, is not new. These systems have been used in the industry for many years. However, such systems are very complex and expensive, and only used in a very small number of special applications where the added performance justifies the cost and complexity.
Therefore, substantial industry-wide savings could be realized if a simple, low cost system or method were available that offers the control and efficiency of a fully metered system without the complexity and cost, and which is simple, reliable, and can be installed without major modifications to the burner and/or the structure of the water heater or other heating system. Such a system would provide a practical and economical alternative means of improving the efficiency of countless water heating and other types of heating systems in use.